1. Field of the Invention
The present invention relates to golf balls. Specifically, the present invention relates to processes for designing a rugged pattern on a golf ball surface.
2. Description of the Related Art
Golf balls have a large number of dimples on the surface thereof. The dimples disturb the air flow around the golf ball during flight to cause turbulent flow separation. By causing the turbulent flow separation, separation points of the air from the golf ball shift backwards leading to a reduction of drag. The turbulent flow separation promotes the displacement between the separation point on the upper side and the separation point on the lower side of the golf ball, which results from the backspin, thereby enhancing the lift force that acts upon the golf ball. The reduction of drag and the enhancement of lift force are referred to as a “dimple effect”.
The ratio of the total area of dimples to the surface area of a phantom sphere of a golf ball is referred to as occupation ratio. It is known that the occupation ratio correlates to flight performance. A golf ball having an increased occupation ratio is disclosed in JP4-347177. The golf ball has circular dimples.
In a golf ball in which a small circular dimple is arranged in a zone surrounded by a plurality of large circular dimples, a high occupation ratio can be achieved. However, the small dimple does not contribute to the flight performance of the golf ball. There is a limit to the dimple effect of the golf ball having the circular dimples.
U.S. Pat. No. 7,198,577 discloses a golf ball having hexagonal dimples. The occupation ratio of the golf ball is high. The golf ball does not have any small dimples. In the golf ball, the dimples are orderly arranged. The dimple effect of the golf ball is insufficient.
The United States Golf Association (USGA) has established the rules about symmetry of golf balls. According to the rules, the trajectory during PH (pole horizontal) rotation and the trajectory during POP (pole over pole) rotation are compared with each other. A golf ball having a large difference between these two trajectories does not conform to the rules. In other words, a golf ball having inferior aerodynamic symmetry does not conform to the rules. A golf ball with inferior aerodynamic symmetry has a short flight distance because the aerodynamic characteristic of the golf ball for PH rotation or for POP rotation is inferior. The rotation axis for PH rotation extends through the poles of the golf ball, and the rotation axis for POP rotation is orthogonal to the rotation axis for PH rotation.
Generally, a golf ball is formed by a mold including upper and lower mold halves. The mold has a parting line. A golf ball obtained by this mold has a seam at a position along the parting line. Through this forming, spew occurs along the seam. The spew is removed by means of cutting. By cutting the spew, the dimples near the seam are deformed. In addition, the dimples near the seam tend to be orderly arranged. The position of the seam coincides with the position of the equator or the seam is located near the equator. The region near the equator is a unique region. The cutting of the spew impairs the aerodynamic symmetry of the golf ball.
The present inventor has proposed a process for designing a rugged pattern using a Voronoi tessellation in Japanese Patent Application No. 2012-244973. In this designing process, a pattern having a high occupation ratio can be obtained. In addition, in this designing process, a pattern having distortional dimples can be obtained. In the pattern having distortional dimples, the dimples are not orderly arranged. The golf ball has excellent flight performance.
In the pattern obtained by the Voronoi tessellation, the effect of deformation of dimples by cutting of spew prominently appears. Specifically, a phenomenon appears that the aerodynamic symmetry is impaired. In addition, when the amount of cutting is varied, a phenomenon also appears that the height of a trajectory during PH rotation is varied considerably. The reason for these phenomena is thought to be that the volumes of the dimples adjacent to the seam are reduced considerably by the cutting.
An object of the present invention is to provide a golf ball having excellent flight performance. Another object of the present invention is to provide a golf ball having excellent aerodynamic symmetry.